OBJECTIVE To assess whether plasma endothelin, adrenaline, noradrenaline, arginine vasopressin, adrenocorticotropin, and cortisol concentrations were higher during cardiopulmonary resuscitation in patients in whom resuscitation was successful than in those in whom it failed, and to measure the concentrations of these hormones in the immediate post-resuscitation phase. METHODS Prospective, descriptive study. METHODS Emergency medical service at a university hospital. METHODS 60 patients with cardiac arrest out of hospital. METHODS Blood samples were drawn and blood pressure and heart rate were measured during cardiopulmonary resuscitation, before and after the first dose of adrenaline was given and at 5, 15, 30, and 60 minutes after the restoration of spontaneous circulation. Plasma hormone concentrations were measured by radio-immunoassays. RESULTS 24 of the 60 patients were successfully resuscitated and admitted to hospital: 36 were not. During cardiopulmonary resuscitation before adrenaline was given, the plasma concentration of endothelin (mean (SEM)) in resuscitated and in not resuscitated patients was 4.3 (0.9) pg/ml and 5.5 (0.4) pg/ml respectively (NS), adrenaline was 14.1 (2.0) ng/ml and 25.3 (3.6) ng/ml (P < 0.01), noradrenaline was 5.0 (0.9 ng/ml) and 8.4 (1.1 ng/ml) (P < 0.05), arginine vasopressin was 193 (28) pg/ml and 70 (9) pg/ml (P < 0.001), adrenocorticotropin was 128 (34) pg/ml and 57 (6) pg/ml (P < 0.05), and cortisol was 18 (3) microgram/dl and 15 (2) microgram/dl (NS). During cardiopulmonary resuscitation after adrenaline was given endothelin in resuscitated and in not resuscitated patients was 4.0 (1.0) pg/ml and 5.3 (0.5) pg/ml (NS), adrenaline was 145 (16) ng/ml and 201 (21) ng/ml (P < 0.05), noradrenaline was 3.9 (0.9) ng/ml and 8.3 (1.1) ng/ml (P < 0.01), arginine vasopressin was 177 (27) pg/ml and 58 (9) pg/ml (P < 0.001), adrenocorticotropin was 234 (92) pg/ml and 85 (9) pg/ml (P < 0.001), and cortisol was 17 (2) microgram/dl and 13 (2) microgram/dl (NS). CONCLUSIONS Despite a tremendous adrenosympathetic response, the lower arginine vasopressin and adrenocorticotropin concentrations during cardiopulmonary resuscitation in patients in whom resuscitation failed may influence vital organ perfusion and hence the success of resuscitation. Plasma concentrations of arginine vasopressin and adrenocorticotropin may have a more important effect on outcome than previously thought.